DEVELOPMENT AND REPRODUCTION 



7 



stem. The difference between plants of Jerusalem artichoke {Helianthus 

 tuberosus) grown under these two sets of conditions was very striking (Fig. 163). 

 In this case the lowland form was tall, with spirally arranged leaves, and the 

 whole plant was very similar to the common sunflower {Helianthus annuus), but 

 the alpine plant, grown at an altitude of 2300 m., was quite different in appear- 

 ance, being very much smaller, with almost no stem and with the leaves in a 

 rosette close to the ground. This species is thus so strongly influenced by the 

 climatological conditions of high altitudes that it assumes the typical form of 

 an alpine plant even in the first generation under these conditions. 



These examples show how readily the forms of many plants become altered 

 by changed external conditions. Even in 

 the first generation this influence of the 

 surroundings may be very marked, and 

 when the new set of conditions is effective 

 throughout a number of generations the 

 resulting changes may be inherited. Such 

 inherited characteristics may then be 

 retained throughout a number of genera- 

 tions, notwithstanding further environ- 

 mental changes. 1 Fig. 164 represents 

 spruce seedlings three years old, all grown 

 under identical conditions, but from seed 

 that came from different regions. Seed 

 from trees growing under favorable con- 

 ditions, at relatively low altitude (800 

 m.), produced very large plants (Fig. 

 164, 2), but seed from trees growing in 

 the same geographic region but at higher 

 altitude (1600 m.) produced much smaller 

 plants (Fig. 164, 1). The plants obtained 

 from seed that grew in Finland (Fig. 164, 3) 

 were much smaller than any of the others. 



Practical as well as theoretical impor- 

 tance is attached to the principle of 

 heredity just illustrated, for to obtain a 

 good agricultural crop not only must the 

 soil be well cultivated and fertilized but seed of a good stock or strain must 

 be used also. 



Scientists have not been satisfied with studying the influence of external 

 conditions in the control of form and structure of plants, but they have also 

 been interested in discovering the genetic relationships that exist between 

 plant organs. Until very recently problems of this sort have been attacked 

 exclusively by the method of simple observation. From such morphological ob- 

 servations, the plant body (in the case of vascular plants) is considered as made 



1 Demoore, J., La memoire organique. Bull. Soc. Roy. Sci. Med. et Nat. Bruxelles 65 : 28-40. 1907. 



Fig. 165. — Formation of potato 

 tubers above the soil, on darkened por- 

 tion of the stem. {After Vochting.) 



